The present invention is related to a process for formulating divalproex sodium solid oral dosage forms. The process comprises preparing a neutralized divalproex sodium solution, wherein the valproic acid moiety of the divalproex sodium is neutralized by addition of a strong base. The neutralized divalproex sodium solution is subsequently processed into a solid dosage form, such as divalproex sodium tablets.
Valproic acid, or 2-propylpentanoic acid, and its salts and derivatives are compounds with anticonvulsant properties. Of these, valproic acid and its sodium salt (sodium valproate) are the most well known. U.S. Pat. No. 3,325,361 describes the use of valproic acid, sodium valproate and other salts and derivatives of valproic acid as anti-convulsants.
It has been recognized by those skilled in the art that both valproic acid and sodium valproate are difficult to formulate into solid oral dosage forms. Valproic acid, for example, is an oily liquid. Sodium valproate is known to be very hygroscopic and to liquify rapidly, and is, therefore, difficult to formulate into tablets.
Efforts have been made to address the problems associated with formulating valproic acid and sodium valproate into solid oral dosage forms. U.S. Pat. No. 5,049,586 (Ortega, et al.) describes valproic acid tablets having a specific composition, which tablets are said to be stable. The tablets contain valproic acid, magnesium oxide, corn starch, poyvinylpyrrolidone, sodium carboxymethylcellulose, and magnesium stearate in specific proportions.
U.S. Pat. No. 5,017,613 (Aubert, et al.) describes a process for preparing a composition containing valproic acid in combination with valproate sodium, wherein the process does not use any binder or granulating solvent. In the process, a mixture of valproic acid and ethylcellulose is prepared and valproate sodium is added to the mixture to form drug granules in the absence of any binder or granulating solvent. Precipitated silica is added to the granules before the compression into tablets.
Efforts have also been made to overcome the limited utility of valproic acid and sodium valproate in formulating solid dosage forms by creating a different salt form or a derivative of valproic acid. U.S. Pat. No. 4,895,873 (Schafer) describes a crystalline calcium salt of valproic acid, in which five valproic acid radicals are associated with one calcium ion. The crystalline salt, called calcium pentavalproate, is said to be non-hygroscopic.
U.S. Pat. No. 4,558,070 (Bauer, et al.) describes potassium, cesium or rubidium salt of valproic acid, which is prepared by combining 4 moles of valproic acid with 1 mole of the potassium, cesium or rubidium. U.S. Pat. No. 4,699,927 (Deboeck) describes arginine, lysine, histidine, ornithine or glycine salts of valproic acid.
U.S. Pat. Nos. 5,212,326 and 4,988,731 (Meade) describe divalproex sodium and its preparation. Divalproex sodium is described as an ionic oligomer in which one mole each of the valproic acid form coordinate bonds with the sodium of the sodium valproate molecule, where the valproate ion is ionically bonded to the sodium ion. Meade also describes the oligomeric compound as having better physical properties than either monomer from which it is made in that the oligomer is a crystalline, non-hygroscopic, stable solid compound.
Some patents describe sustained release dosage forms for divalproex sodium, valproic acid, its salts, amides, or other derivatives. U.S. Pat. No. 5,980,943 (Ayer, et al.) describes a sustained release delivery device for administering divalproex sodium, valproic acid, and its salts and derivatives. The device comprises a semipermeable wall containing drug granules that are microencapsulated with polyalkylene oxide or carboxymethylcellulose polymer.
U.S. Pat. No. 4,913,906 (Friedman, et al.) describes a controlled release dosage form containing divalproex sodium, valproic acid, valpromide and other valproic acid salts and derivatives. The composition is prepared by mixing the drug with hydroxypropyl cellulose, ethylcellulose, or esters of acrylic and methacrylic acid, and by applying high pressure to the mixture of the ingredients.
U.S. Pat. No. 5,807,574 (Cheskin, et al.) describes a controlled release dosage form containing divalproex sodium and a process for its preparation. The process involves melting divalproex sodium and mixing it with a molten wax to form a divalproex sodium-wax composite. The drug-wax mixture is formulated into a capsule.
U.S. Pat. No. 5,169,642 (Brinker, et al.) describes a sustained release dosage form containing granules of divalproex sodium, valproic acid or amides or esters or salts thereof and a polymeric viscosity agent. The drug is coated with a sustained release composition comprising specified portions of ethylcellulose or a methacrylic methylester, plasticizer, and detactifying agent.
U.S. Pat. No. 5,068,110 (Fawzi, et al.) describes various delayed-release tablets and capsules currently marketed, including the delayed-release divalproex sodium tablets manufactured by Abbott Laboratories, and states that the stability of an enteric coated capsules is increased by the application of thicker, higher levels of the enteric coating having a thickness of 14 mg/cm2 to 24 mg/cm2, alone or in combination with a hydroxypropylcellulose, hydroxymethylcellulose or hydroxypropylmethyl cellulose coating.
Divalproex sodium is a oligomer having a 1:1 molar ratio of sodium valproate and valproic acid. The oligomer is described as a stable crystalline solid and is designated as sodium hydrogen bis (2-propyl pentanoate).
Upon administration, divalproex dissociates into valproate ion in the gastrointestinal tract, and in that form exerts its pharmacological effect. Divalproex sodium is indicated for the treatment of patients with complex partial seizures, as well as for the treatment of mania associated with bipolar disorders and for prophylaxis of migraine headaches.
U.S. Pat. No. 4,558,070 (Bauer, et al.) indicates that divalproex sodium is a highly stable, non-hygroscopic, crystalline compound. Bauer also discusses a theory behind the stability of divalproex sodium, stating that it is not a mixture of the two precursors but a chemical entity, and that in the oligomer, the outer shell of electrons of the sodium atom is filled by coordination to the oxygen atoms of both valproic acid and valproate ions, resulting in a stable complex where the sodium ion is completely surrounded by oxygen. Bauer, et al., therefore, appears to indicate that the particular oligomeric structure and the molar ratio of divalproex sodium accounts for the stability of the compound.
Applicants have discovered that divalproex sodium may be formulated into stable solid oral dosage forms, even in the absence of the oligomeric structure and the equimolar ratio of sodium valproate and valproic acid.
It is an object of the invention to provide a process for preparing a divalproex sodium composition.
It is a further object of the invention to provide a process for preparing a divalproex sodium composition, wherein the process comprises preparing a neutralized divalproex sodium solution by combining divalproex sodium, having a valproic acid moiety and a sodium valproate moiety, with a base (e.g., sodium hydroxide) and an aqueous solvent. The base is added in sufficient amount to ensure neutralization of the valproic acid moiety of the divalproex sodium. In the neutralized divalproex sodium solution, divalproex sodium is not present as its oligomeric structure or the 1:1 molar ratio of sodium valproate and valproic acid. The valproic acid of the divalproex sodium is neutralized. Preferably the neutralized divalproex sodium solution contains from about 20 to about 60% valproic acid activity.
It is a further object of the invention to provide an oral solid dosage form containing a therapeutically effective amount of divalproex sodium wherein the divalproex sodium is not present as an oligomeric structure or a 1:1 molar ratio of sodium valproate to valproic acid. It is a further object to provide a new divalproate formulation which provides a delayed release of valproate ion when the dosage form is orally administered to human patients, which dosage form is bioavailable and provides a therapeutic effect which is considered bioequivalent to delayed release divalproex sodium tablets, manufactured by Abbot Laboratories (Depakote(copyright)).
The neutralized divalproex sodium solution is sprayed onto a pharmaceutically acceptable carrier, and the resulting mixture may be processed to obtain a divalproex sodium tablet.
In one embodiment of the invention, the pharmaceutically acceptable carrier comprises a plurality of particles of a material such as, for example, anhydrous lactose or microcrystalline cellulose. A granulate is formed by spraying the neutralized divalproex sodium solution onto the carrier. Additional processing steps may then be undertaken to prepare a uniform granulate suitable for formulating into tablets. Sufficient quantities of pharmaceutically necessary tableting excipients may then be admixed with the divalproex granulate, and the resulting mixture may be compressed into tablets.
The divalproex sodium tablets may be coated with an enteric coating to produce delayed-release divalproex sodium tablets. Optionally, a seal coating may also be applied to the tablets before the enteric coating is provided. The enteric coated divalproex sodium tablets may be further overcoated with a film-coating.
In accordance with the invention, the pharmaceutically acceptable carrier may comprise a plurality of inert beads, for example, sugar beads or nonpareil seeds. The neutralized divalproex sodium solution is sprayed onto the inert beads to produce divalproex sodium coated beads, which can then be formulated into solid dosage forms, such as capsules or tablets.
In one embodiment of the invention, the divalproex sodium coated beads may additionally be coated with an enteric coating. In yet another embodiment, a seal coating may be applied to the drug containing beads prior to the application of the enteric coating. After the coatings are applied, the beads may be admixed with sufficient quantities of pharmaceutically necessary tableting excipients. Pharmaceutical tableting excipients include but are not limited to a lubricant, disintegrant, binder, glidant and/or inert diluent. The tablets thus formulated may further be coated with a film-coating.
The invention is further related to a process for preparing divalproex sodium delayed release tablets, wherein the process comprises preparing a neutralized divalproex sodium solution by combining divalproex sodium, having a sodium valproate moiety and a valproic acid moiety, with sodium hydroxide and an aqueous solvent. The base (e.g., sodium hydroxide) is added in sufficient amount to ensure neutralization of the valproic acid moiety of the divalproex sodium. In the neutralized divalproex sodium solution, divalproex sodium does not retain its oligomeric structure or the 1:1 molar ratio of sodium valproate and valproic acid.
The process further comprises spraying the neutralized divalproex sodium solution on a pharmaceutically acceptable carrier and processing the carrier sprayed with the neutralized divalproex sodium solution to obtain divalproex sodium granules. The granules are further processed to obtain divalproex sodium tablet cores, and an enteric coating is applied to the cores to produce divalproex sodium delayed-release tablets. In one example of the invention, a seal coating is applied to the tablet cores prior to the application of the enteric coating. The delayed-release divalproex sodium tablets may also be coated with a film-coating.
In processing the divalproex sodium granules into tablets, as described above, the granules may be admixed with at least one pharmaceutically necessary excipient and compressed into the tablets. Pharmaceutically acceptable excipients include but are not limited to a lubricant, a disintegrant, a binder, a glidant and/or an inert diluent.
The invention is also directed to a method of treating human patients, comprising administering to human patients an effective amounts of the divalproex sodium formulations prepared in accordance with the invention.
The invention is further related to a method of treating complex partial seizures, mania associated with bipolar disorders, and/or migraine headaches in humans comprising orally administering an effective dose of the divalproex sodium formulations prepared in accordance with the invention.
The term xe2x80x9cneutralized divalproex sodium,xe2x80x9d as used in the present invention, refers to divalproex sodium in which the valproic acid moiety has been neutralized by addition of a strong base, e.g., sodium hydroxide. Neutralized divalproex sodium is not an oligomer. Neutralized divalproex sodium also does not exhibit a 1:1 molar ratio of sodium valproate and valproic acid.
Divalproex sodium tablet prepared using neutralized divalproex sodium solution, therefore, does not contain oligomeric divalproex sodium, nor does it exhibit 1:1 molar ratio of sodium valproate and valproic acid.
The present invention provides a process for preparing divalproex sodium solid oral dosage forms, where the process comprises preparing a neutralized divalproex sodium solution by combining divalproex sodium with an aqueous solvent and a base, the base added in sufficient quantities to ensure neutralization of the valproic acid moiety of the divalproex sodium. The pH of the neutralized divalproex sodium solution is preferably 10.8xc2x11.0, most preferably xc2x10.5. In a preferred embodiment, the aqueous solvent is water.
In an embodiment of the invention, the neutralized divalproex sodium solution may be prepared by dissolving divalproex sodium in a basic solution (e.g. sodium hydroxide and water). Additional sodium hydroxide may be added to ensure that the valproic acid moiety of divalproex sodium is neutralized. In a preferred embodiment, additional water is added to the neutralized divalproex sodium solution so that the resulting solution has 20-60%, most preferably 50xc2x13%, valproic acid activity.
In accordance with the present invention, the neutralized divalproex sodium solution is sprayed onto a pharmaceutically acceptable carrier, and the resulting mixture may then be processed to obtain divalproex sodium tablets.
In one embodiment, the pharmaceutically acceptable carrier comprises a plurality of particles of a material that is an inert diluent, and the divalproex sodium solution is sprayed onto the carrier and dried to produce divalproex sodium granules. In another embodiment of the invention, a binder may also be combined with the neutralized divalproex sodium solution and the pharmaceutically acceptable carrier.
In a preferred embodiment of the invention, the neutralized divalproex sodium solution is sprayed onto the pharmaceutically acceptable carrier in a fluid bed processor with a Wurster apparatus. In one embodiment, this process occurs at a product temperature of 42-48xc2x0 C., with a spray rate of 40-80 ml/min. The divalproex sodium granules may then be dried and then sifted using a mesh screen, e.g., with a 16 mesh screen, to produce divalproex sodium granules.
In a preferred embodiment, the neutralized divalproex sodium solution is diluted, e.g., with isopropyl alcohol before it is sprayed onto the carrier.
The base used in the present invention can be any pharmaceutically acceptable base such as sodium carbonate, sodium bicarbonate, sodium phosphate dibasic, sodium phosphate tribasic, sodium citrate, magnesium hydroxide, magnesium carbonate, calcium carbonate, calcium phosphate, sodium hydroxide and mixtures thereof. A preferred base is sodium hydroxide.
Examples of pharmaceutically acceptable carriers include, but are not limited to, calcium phosphate dihydrate, calcium sulfate dihydrate, microcrystalline cellulose, cellulose derivatives, dextrose, lactose, anhydrous lactose, spray-dried lactose, lactose monohydrate, mannitol, starches, sorbitol and sucrose. Further examples of the carrier include hydroxypropylmethylcellulose, hydroxypropylcellulose, methyl cellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyethyleneglycol, cellulose acetate butyrate, hydroxyethyl cellulose, ethyl cellulose, polyvinyl alcohol, polypropylene, dextrans, dextrins, hydroxypropyl-beta-cyclodextrin, chitosan, copolymers of lactic and glycolic acid, lactic acid polymers, glycolic acid polymers, polyorthoesters, polyanyhydrides, polyvinyl chloride, polyvinyl acetate, ethylene vinyl acetate, lectins, carbopols, silicon elastomers, polyacrylic polymers, maltodextrins, fructose, inositol, trehalose, maltose raffinose, and alpha-, beta-, and gamma-cyclodextrins, and suitable mixtures of the foregoing. A preferred pharmaceutically acceptable carrier is anhydrous lactose.
In certain embodiments, optional pharmaceutical excipients are added to the divalproex sodium granules in the process of formulating the granules into tablets. Such pharmaceutical excipients may include but are not limited to a lubricant, disintegrant, binder, glidant and/or diluent.
Examples of lubricants include magnesium stearate, calcium stearate, oleic acid, caprylic acid, stearic acid, magnesium isovalerate, calcium laurate, magnesium palmitate, behenic acid, glyceryl behenate, glyceryl stearate, sodium stearyl fumarate, potassium stearyl fumarate, and zinc stearate.
Suitable disintegrants include crospovidone, alginates, cellulose and its derivatives, clays, polyvinylpyrrolidone, polysaccharides, such as corn and potato starch, dextrins and sugars. Disintegrants, when used in the formulation, facilitates disintegration when the tablet contacts water in the gastrointestinal tract.
Binders, when added to the formulation, promote granulation and/or promote cohesive compact during the direct compression into tablets. Examples of binders include acacia, cellulose derivatives, gelatin, glucose, polyvinylpyrrolidone, sodium alginate and alginate derivatives, sorbitol, and starch. Binders also include hydrophillic cellulose gums, such as methylcellulose and carboxymethylcellulose, and xanthan gum.
Examples of glidants include but are not limited to corn starch, silica derivatives, and talc.
Examples of inert diluents can include, but are not limited to, calcium phosphate dihydrate, calcium sulfate dihydrate, microcrystalline cellulose, cellulose derivatives, dextrose, lactose, anhydrous lactose, spray-dried lactose, lactose monohydrate, mannitol, starches, sorbitol and sucrose. Further examples of the carrier include hydroxypropylmethylcellulose, hydroxypropylcellulose, methyl cellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyethyleneglycol, cellulose acetate butyrate, hydroxyethyl cellulose, ethyl cellulose, polyvinyl alcohol, polypropylene, dextrans, dextrins, hydroxypropyl-beta-cyclodextrin, chitosan, copolymers of lactic and glycolic acid, lactic acid polymers, glycolic acid polymers, polyorthoesters, polyanyhydrides, polyvinyl chloride, polyvinyl acetate, ethylene vinyl acetate, lectins, carbopols, silicon elastomers, polyacrylic polymers, maltodextrins, fructose, inositol, trehalose, maltose raffinose, and alpha-, beta-, and gamma-cyclodextrins, and suitable mixtures of the foregoing. A preferred pharmaceutically acceptable carrier is anhydrous lactose.
The tablet cores described above may be coated with an enteric coating to obtain delayed-release divalproex sodium tablets that remain intact in the stomach and release the active ingredient in the intestine. Suitable enteric coating may comprise cellulose acetate phthalate, polyvinyl acetate phthalate, acrylic resins such as Eudragit L.RTM., shellac, cellulose acetate butyrate, hydroxypropyl methylcellulose phthalate or combinations thereof.
Additional materials suitable for use in the enteric coating include phthalates including cellulose acetyl phthalate, cellulose triacetyl phthalate, sodium cellulose acetate phthalate, cellulose ester phthalate, cellulose ether phthalate, methylcellulose phthalate, cellulose ester-ether phthalate, hydroxy propyl cellulose phthalate, alkali salts of cellulose acetate phthalate, alkaline earth salts of cellulose acetate phthalate, calcium salt of cellulose acetate phthalate, ammonium salt of hydroxypropyl methylcellulose phthalate, cellulose acetate hexahydrophthalate, hydroxypropyl methylcellulose hexahydrophthalate, and polyvinyl acetate phthalate. The enteric materials are discussed in Remington""s Pharmaceutical Sciences, 17th Ed., page 1637 (1985).
The enteric coating may be applied by press coating, molding, spraying, dipping and/or air-suspension or air tumbling procedures. A preferred method of applying the enteric coating is by pan coating, where the enteric coating is applied by spraying the enteric composition onto the tablet cores accompanied by tumbling in a rotating pan. The enteric coating material may be applied to the tablet cores by employing solvents, including an organic, aqueous or a mixture of an organic and aqueous solvent. Examplary solvents suitable in applying the enteric coating include an alcohol, ketone, ester, ether, aliphatic hydrocarbon, halogenated solvents, cycloaliphatic solvents, aromatic, heterocyclic, aqueous solvents, and mixtures thereof. In a preferred embodiment, the enteric coating comprises cellacefate and diethyl phthalate in isopropyl alcohol and acetone. In preferred embodiments, the coating has a thickness from about 6% to about 8% of the final dosage form.
In accordance with the invention, the divalproex sodium tablet cores may further be coated with a seal coating. In a preferred embodiment, the seal coating occurs between the tablet core and the enteric coating. The seal coating may comprise a hydrophilic polymer. Examples include but are not limited to hydroxypropyl cellulose, hydroxypropylmethylcellulose, methoxypropyl cellulose, hydroxypropylisopropylcellulose, hydroxypropylpentylcellulose, hydroxypropylhexylcellulose and any mixtures thereof.
The seal coating, like the enteric coating, may be applied by press coating, molding, spraying, dipping and/or air-suspension or air tumbling procedures. A preferred method of applying the seal coating is by pan coating, where the seal coating is applied by spraying it onto the tablet cores accompanied by tumbling in a rotating pan. The seal coating material may be applied to the tablets as a suspension by employing solvents, e.g., an organic, aqueous, or a mixture of an organic and aqueous solvent. Examplary solvents suitable in applying the seal coating include aqueous-based solutions, an alcohol, ketone, ester, ether, aliphatic hydrocarbon, halogenated solvents, cycloaliphatic solvents, aromatic, heterocyclic, aqueous solvents, and mixtures thereof. In a preferred embodiment, the seal coating comprises hydroxypropyl cellulose and hydroxypropylmethylcellulose, and is delivered as a suspension using ethanol as a solvent.
The divalproex sodium tablets may be overcoated with a pharmaceutically acceptable film coating, e.g., for aesthetic purposes (e.g., including a colorant), for stability purposes (e.g., coated with a moisture barrier), for taste-masking purposes, etc. For example, the tablets may be overcoated with a film coating, preferably containing a pigment and a barrier agent, such as hydroxypropylmethycellulose and/or a polymethylmethacrylate. An example of a suitable material which may be used for such overcoating is hydroxypropylmethylcellulose (e.g., Opadry(copyright), commercially available from Colorcon, West Point, Pa.). In a preferred embodiment, an overcoating is applied to the divalproex sodium tablets that have already been coated with a seal coating and an enteric coating. The overcoat may be applied using a coating pan or a fluidized bed, and may be applied by using a solvent, preferably an aqueous solvent.
The final product is optionally subjected to a polishing step to improve the appearance of the final product and also to facilitate the manipulation of the formulation post manufacture. For example, the slippery nature of the polished dosage form aids in filling printer carrier bars with the formulation and facilitates final packaging of the product. Suitable polishing agents are polyethylene glycols of differing molecular weight or mixtures thereof, talc, surfactants (e.g., Brij types, Myrj types, glycerol mono-stearate and poloxamers), fatty alcohols (e.g., stearyl alcohol, cetyl alcohol, lauryl alcohol and myristyl alcohol) and waxes (e.g., carnauba wax, candelilla wax and white wax). Preferably, polyethylene glycols having molecular weight of 3,000-20,000 are employed.
In certain embodiments of the present invention, the pharmaceutically acceptable carrier onto which the neutralized divalproex sodium solution is sprayed comprises a plurality of inert beads, e.g., sugar beads. The divalproex sodium coated beads thus obtained may be coated with an enteric coating. The beads may also be coated with a seal coating, preferably the seal coating being applied before the enteric coating. The suitable enteric coating and the seal coating materials are set forth above.
The divalproex sodium beads may be formulated into solid oral dosage forms. For example, the beads made be formulated into tablets by admixing them with sufficient quantities of a pharmaceutically necessary tableting excipient and compressing the resulting mixture. The pharmaceutically necessary tableting excipient is selected from the group consisting of a lubricant, a disintegrant, a binder, a glidant, an inert diluent and mixtures thereof. Suitable tableting excipients are set forth above.
In certain preferred embodiments, the present invention provides a process for preparing divalproex sodium delayed-release tablets. The process comprises preparing a neutralized divalproex sodium solution by combining divalproex sodium, having a sodium valproate moiety and a valproic acid moiety, with an aqueous solvent and a base, e.g., sodium hydroxide, the bases being added in sufficient amount to ensure neutralization of the valproic acid moiety of the divalproex sodium. The process further comprises spraying the neutralized divalproex sodium solution onto a pharmaceutically acceptable diluent, processing the resulting mixture to obtain divalproex sodium granules, and processing the granules to obtain tablet cores. An enteric coating is applied to the divalproex sodium tablet cores to produce divalproex sodium delayed-release tablets. Preferably, the delayed-release tablet further comprises a seal coating, applied between the core and the enteric coating. Suitable material for the seal coating and the enteric coating, as well as the procedures for application of these coatings, are set forth above. The tablet thus produced does not contain divalproex sodium that is an oligomeric compound and does not have a 1:1 molar ratio of sodium valproate and valproic acid. Rather, the tablets of the present invention contain divalproex sodium in which the valproic acid moiety has been neutralized.
The pH of the neutralized divalproex sodium solution is preferably about 10.8xc2x10.5, and the neutralized divalproex sodium solution preferably has about 50xc2x13% valproic acid activity. A preferred aqueous solvent for preparation of the neutralized divalproex sodium solution is water.
In a preferred embodiment, the processing of the divalproex sodium granules to obtain tablets comprises drying and then screening the divalproex sodium granules, and admixing the screened divalproex sodium granules with pharmaceutically necessary excipients and compressing the resulting mixture into tablets. The pharmaceutically acceptable excipients are selected from the group consisting of a lubricant, a disintegrant, a binder, a glidant, an inert diluent and mixtures thereof. Examples of suitable excipients are listed above.
In a preferred embodiment, the neutralized divalproex sodium solution is diluted with isopropyl alcohol before it is sprayed onto anhydrous lactose in a fluid bed processor with a Wurster apparatus at product temperature of, e.g., 42-48xc2x0 C. and a spray rate of, e.g., 40-80 ml/min to form granules. The granules are sized through an appropriate sized screen, e.g., a 16 mesh screen. The sized granules are blended with crospovidone, anhydrous lactose, colloidal silicon dioxide and magnesium stearate and compressed into tablets. The tablets are coated with a seal coating in a coating pan with a suspension of hydroxypropylmethylcellulose, hydroxypropylethylcellulose, hydroxypropyl cellulose and magnesium stearate in ethanol. An enteric coating is then applied, also in a coating pan. The enteric coating comprises cellacefate and diethyl phthalate in isopropyl alcohol and acetone. As an optional final step, the enteric coated tablet is film coated and subjected to a polishing step.